link_to_original_img_on_index
Long ago, makethumbs didn't create slideshow pages, so the index page had a link to the original image and to the reduced image. These days the thumbnails are linked to the slideshow pages, and that's where most people browsing should head. So there isn't much reason to link to the original image directly from the index page. But the option is there for anyone who still wants that link.
link_to_original_img_on_index
is a boolean variable;
set it to 1 to have links to your original images
on your index page.